- Email: [email protected]
Visible light-operated saccharide–O2 biofuel cell based on the photosensitization of chlorophyll derivative on TiO2 film
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33 (2008) 2845 – 2849
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Visible light-operated saccharide–O2 biofuel cell based on the photosensitization of chlorophyll derivative on TiO2 film Yutaka Amao, Yumi Takeuchi Department of Applied Chemistry, Oita University, Dannoharu 700, Oita 870-1192, Japan
art i cle info
ab st rac t
Article history:
The visible light-operated saccharide–O2 biofuel cell consisting of zinc chlorin-e6 (ZnChl-e6)
Received 7 December 2007
adsorbed on nanocrystalline TiO2 layer coated onto optical transparent conductive glass
Received in revised form
electrode (OTE) as an anode, platinum-coated OTE as a cathode, and the fuel solution
19 March 2008
containing sucrose as a saccharide, invertase, glucose dehydrogenase (GDH) and NAD+ is
Accepted 20 March 2008
studied as a new type biofuel cell. The short-circuit photocurrent (ISC) and the open-circuit
Available online 8 May 2008
photovoltage (VOC) of this cell are 9.0 mA cm2 and 415 mV, respectively. The peaks in the
Keywords: Biofuel cell Bioresource Chlorophyll Titanium oxide Photosynthesis
1.
photocurrent action spectrum of this cell are observed at 400 and 800 nm and the incident photon-to-current efficiency (IPCE) values at 400 and 800 nm are estimated to be ca. 17.3% and 10.6%. Thus, a new type of visible light-operated saccharide–O2 biofuel cell with the visible and near IR photosensitization of ZnChl-e6 molecules on nanocrystalline TiO2 film electrode is accomplished. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
Introduction
Energy utilization of the biomass resources is important in the environmental science and the development of energy source research fields [1–3]. Some renewable biomass resources based on the saccharides are starch, cellulose, sucrose, lactose and so on. These oligo- and poly saccharides are hydrolyzed to form monosaccharides such as glucose. The conversion of glucose to hydrogen will be a useful biomass resources utilization. Some studies on the hydrogen production from glucose using the enzymatic pathway have been reported [4–6]. The hydrogen production from glucose with the combination of the glucose dehydrogenase (GDH) and the hydrogenase, which catalyzes the proton reduction, has been reported [7]. Visible light-induced hydrogen production systems from the glucose are also developed with the photosensitizer such as zinc porphyrin and Mg chlorophyll-a in the
reserved.
above-mentioned system [8]. We previously reported the enzymatic photo-induced hydrogen production from sucrose using the photosensitization of Mg or Zn chlorophyll-a as shown in Scheme 1 [9,10]. On the other hand, the biofuel cells using biomass have attracted much attention in recent years [11]. By developing these photo-induced hydrogen production systems in a photoelectrochemical conversion system, a new type of photo-operated saccharide–O2 biofuel cell can be developed. Many studies on the photoelectrochemical conversion system using photosynthesis dyes chlorophyll or its derivative assembled electrode have been reported [12–16]. In the photoelectrochemical conversion system, dye-sensitized nanocrystalline TiO2 cells have attracted much attention as low cost conventional solid state photovoltaic conversion devices for visible light utilization [17–20]. In dye-sensitized nanocrystalline TiO2 cells, TiO2 acts as an electron accepter.
Corresponding author. Fax: +81 97 554 7972.
E-mail address: [email protected] (Y. Amao). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.03.034
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Scheme 1 – Visible light-induced hydrogen production system coupling the sucrose hydrolysis with invertase and GDH and hydrogen production with colloidal platinum using the photosensitization of chlorophyll-a (MChl-a; M ¼ Zn or Mg) in the presence of methylviologen (MV2+). As the dye molecule adsorbed TiO2 shows no photocatalytic activity under visible light region, the chemical components are not decomposed with TiO2. Thus, the dye molecule adsorbed nanocrystalline TiO2 electrode also is useful for the photo-operated saccharide-O2 biofuel cell. We previously reported the photo-operated biofuel cell using chlorophyll derivative adsorbed on nanocrystalline TiO2 layer coated onto optical transparent conductive glass electrode (OTE) as an anode, platinum-coated OTE as a cathode, and the solution containing glucose invertase, (GDH) and NAD+ as a fuel [21]. In the view point of biomass utilization, however, the development of photo-operated biofuel cell using oligo- and polysaccaride is desirable. In this photo-operated biofuel cell, glucose formation is one of the most important processes. As mentioned above, we reported the enzymatic photo-induced hydrogen production with the system combination of sucrose hydrolysis with enzyme invertase and the photoredox reaction of Mg or Zn chlorophyll-a, methylvilogen (MV2+) and platinum colloid as shown in Scheme 1 [9,10]. Thus, it is necessary to combine the glucose formation based on the oligo- and polysaccaride hydrolysis with enzyme in photo-operated biofuel cell. In this paper, we describe the visible light-operated saccharide–O2 biofuel cell consisting of zinc chlorin-e6 (ZnChl–e6) adsorbed on nanocrystalline TiO2 layer coated onto OTE as an anode, platinum-coated OTE as a cathode, and the solution containing sucrose, which is most simple oligo saccharide, invertase, GDH and NAD+ as a fuel as shown in Fig. 1.
2.
Experimental section
2.1.
Materials
Chl-e6, GDH from Bacillus sp. and invertase from Yeast are purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan). NAD+ and NADH are purchased from Oriental Yeast Co. Ltd. Hydrogen hexachloplatinate hexahydrate is obtained from Kanto Chemical Co. Ltd (Tokyo, Japan). Titanium dioxide powder (P25) is purchased from Degussa. OTE (10–15 O/square SnO2: fluorine coated) is obtained from Nihon Sheet Glass Co. Ltd. The other chemicals are analytical grade or the highest grade available. One unit of GDH activity is defined as the amount of enzyme that reduced 1.0 mmol NAD+ to NADH by glucose per min. One unit of invertase activity is defined as the amount of enzyme that produced 1.0 mmol glucose by sucrose per min.
Fig. 1 – Visible light-operated sucrose–O2 biofuel cell based on the combination of NAD+ reduction with enzymatic sucrose hydrolysis photosensitization of ZnChl-e6 on nanocrystalline TiO2 layer-coated OTE (anode) and the electrochemical reduction to the water of oxygen on to the platinum-coated OTE (cathode).
2.2.
Synthesis of ZnChl-e6
ZnChl-e6 is synthesized by refluxing Chl-e6 with about 10 times the molar equivalent of zinc acetate in 100 ml methanol at 60 1C for 2 h. The synthesis of ZnChl-e6 is monitored by UV–vis absorption spectrum using Shimadzu Multispec 1500 spectrophotometer. During the reaction, the characteristic absorption bands of ZnChl-e6 at 418 and 638 nm increase and the absorbance at 400, 514 and 660 nm of Chl-e6 decrease gradually. After the mixture is cooled to room temperature, the solvent is removed under vacuum and then the reaction mixture is washed with water to remove the unreacted zinc acetate dihydrate. Finally, ZnChl-e6 is obtained in water as a precipitation. ZnChl-e6 is collected by filtration and washed with water. The purification is performed by recrystallization from water–methanol (5:1) solution.
2.3.
Preparation of TiO2 film electrode
The nanocrystalline TiO2 film is prepared by a similar procedure to that described in the literature [22]. TiO2 powder is dispersed by grinding water and HNO3 aqueous solution. The viscous suspension is spread onto OTE (1 5 cm2) at room temperature using scotch tape as a spacer. A thin film is obtained by raking off the excess of suspension with a glass rod. After the tape is removed and the plate is dried using hot plate at 80 1C for 30 min, this plate is annealed at 450 1C for
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30 min under ambient condition to form a nanocrystalline TiO2 film electrode. The thickness of the film, determined by using a micron-sensitive calliper, is about 10 mm.
2.4. Preparation of ZnChl-e6 adsorbed nanocrystalline TiO2 electrode ZnChl-e6 adsorbed nanocrystalline TiO2 electrode is prepared as follows. An OTE glass plate with a nanocrystalline TiO2 film is dipped into 0.2 mmol dm3 ZnChl-e6 in methanol solution at room temperature for 6 h. After dipping, the plate is washed with methanol several times and then the plate is dried under vacuum overnight.
2.5.
Preparation of platinum-coated OTE electrode
The platinum-coated OTE electrode is prepared by thermal decomposition of hydrogen hexachloplatinate hexahydrate from 2-propanol solution on an OTE substrate as the following method. An OTE glass plate is dipped into 1.0 mmol dm3 hydrogen hexachloplatinate hexahydrate in 2-propanol solution at room temperature for 30 min. After dipping, the plate is dried at room temperature for 30 min under ambient condition and then is annealed at 380 1C for 30 min under ambient condition. The active area of electrode is 1.0 cm2.
2.6. Characterization of visible-operated sucrose–O2 biofuel cell using ZnChl-e6 adsorbed on nanocrystalline TiO2 film Photocurrent–photovoltage characteristic of the ZnChl-e6 adsorbed on TiO2 electrode is measured with a sandwichtype cell. The working electrode with the ZnChl-e6 adsorbed on TiO2 film is gently squeezed together with a platinumcoated OTE glass electrode (counter electrode) using spring and irradiated from the substrate side of working electrode. The anodic solution consists of 0.1 mol dm3 sucrose, five units invertase, 3.5 mmol dm3 NAD+, five units GDH and 0.1 mol dm3 KCl in 50 mmol dm3 in 50 ml of 10 mmol dm3 potassium phosphate buffer (pH 7.0). The dissolved oxygen in anodic solution is used as cathodic reaction of oxygen to water. A solar simulator (YSS-40, Yamashita Denso) is used as a light source (A.M. 1.5 100 mW cm2) for the photocurrent and photovoltage characteristics with the two digital multimeter with model 2000-J (Keithley) as a current meter and model 34401A (Agilent) as a voltage meter, respectively. To prevent the direct photoexcitation of TiO2, wavelengths of less than 420 nm were blocked with a cut-off filter (SCF-50S-42L Sigma Koki Co., Ltd.). The photocurrent and photovoltage are changed using 500-O variable resistor. The active electrode area is typically 0.25 cm2. The fill factor (FF) is defined by FF ¼ IPhðmaxÞ VPhðmaxÞ =ISC VOC
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2847
defined by the following equation: Z ¼ ISC VOC FF=Pin
(2)
Here Pin is the power of incident white light. A 400 W xenon lamp with a monochromator is used as light source for photocurrent action spectra measurements. The cell is operated in the short-circuit mode. As the ZnChl-e6 has the absorption band in the region between 400 and 900 nm, the incident photon-to-current conversion efficiency (IPCE) values are determined in this region. The IPCE is then calculated according to the following equation: IPCE ¼ 1240iph ðmAÞ=PðmWÞlðnmÞ
(3)
where iph and P are the photocurrent and power of the incident radiation per unit area and l is the wavelength of the monochromatic light. The maximums and the minimum values of light intensity between 400 and 900 nm are 110 and 24 mW cm2, respectively.
3.
Results and discussion
3.1.
Photocurrent–photovoltage characteristics
Fig. 2 shows the photocurrent–photovoltage characteristic of a sandwich cell irradiated with 100 mW cm2. The shortcircuit photocurrent (ISC) is 9.0 mA cm2, and the open-circuit photovoltage (VOC) is 415 mV, respectively. The power output of cell as a function of photovoltage is shown in Fig. 3. The maximum power is estimated to be 0.028 mW cm2. FF and Z values calculated by Eqs. (1) and (2) are 7.4 104 and 0.000028%, respectively. In contrast, ISC and VOC under the dark condition are ca. zero. Thus, this cell is operated with the visible-light photosensitization of nanocrystalline TiO2 films by ZnChl-e6. We already reported the photo-operated glucose–O2 biofuel cell. In the photo-operated glucose–O2 biofuel
(1)
where IPh(max) and VPh(max) are the photocurrent and photovoltage for maximum power output (Pmax) and ISC and VOC are the closed-circuit photocurrent and open-circuit photovoltage. The overall photoenergy conversion efficiency (Z) is
Fig. 2 – Photocurrent–photovoltage characteristic sucrose–O2 biofuel cell under 100 mW cm2 condition.
of
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Fig. 4 – Photocurrent action spectra of sucrose–O2 biofuel cell as a function of wavelength (400–900 nm). Fig. 3 – Power output of sucrose–O2 biofuel cell as a function of photovoltage under light intensity of 100 mW cm2 condition.
cell, FF and Z values are 4.0 103 and 0.0021%, respectively. Thus, the cell performance slightly decreases by combination of sucrose hydrolysis in photo-operated biofuel cell. To investigate the effect of oxygen concentration on the photocurrent–photovoltage characteristics of the cell, the oxygen concentration in the anodic solution is changed. By using the argon-saturated anodic solution, the VOC ( ¼ 420 mV) value is almost the same as that of using the anodic solution including dissolved oxygen. On the other hand, the ISC value is gradually decreased with irradiation time under argonsaturated condition. In contrast, the VOC and ISC values slightly increase using the oxygen-saturated anodic solution. These results show that photo-operated sucrose–O2 biofuel cell based on the visible-light photosensitization of nanocrystalline TiO2 films by ZnChl-e6 is developed. However, the conversion efficiency of photon-to-current is still low.
3.2.
metal-free and ZnChl-e6 adsorbed on a nanocrystalline TiO2 film electrode. The photocurrent action spectrum of cell using metal-free Chl-e6 is similar to that of the UV–vis absorption spectrum in methanol solution and the maximum peak of photocurrent action and UV–vis spectra is 670 nm [23]. However, the photocurrent action spectrum of cell using ZnChl-e6 is shifted to near IR region compared with that of the UV–vis absorption spectrum in methanol solution and the maximum peak of photocurrent action and UV–vis spectra is 780 nm [24]. This result indicates that ZnChl-e6 molecules are aggregated on a nanocrystalline TiO2 film electrode and the absorption band is shifted to near IR region. This result indicates that ZnChl-e6 molecules are aggregated such as bacteriochlorophyll dimer on a nanocrystalline TiO2 film electrode and the absorption band is shifted to red region. Thus, the peak of photocurrent action spectrum of cell based on the visible-light photosensitization of nanocrystalline TiO2 films by ZnChl-e6 is shifted to red region compared with that of UV–vis absorption spectrum in methanol solution.
Photocurrent action spectrum
Fig. 4 shows the photocurrent action spectrum of a sandwich cell, where the IPCE is plotted as a function of wavelength. The maximum peaks in the photocurrent action spectrum of the cell are observed at 400 and 800 nm and the IPCE values at 400 and 800 nm are estimated to be ca. 17.3% and 10.6%. In the photo-operated glucose–O2 biofuel cell, the IPCE values at 400 and 800 nm are estimated to be ca. 7.5% and 5.0%. Thus, the IPCE values at 400 and 800 nm slightly increase by a combination of sucrose hydrolysis in the photo-operated biofuel cell. In contrast, the maximum peaks of UV–vis absorption spectrum in methanol solution are 413 and 638 nm. The peak in the photocurrent action spectrum is shifted to red region compared with that of UV–vis absorption spectrum obtained in a methanol solution. We previously reported the solar cell based on the photosensitization of
4.
Conclusion
In this work, visible light-operated sugar–O2 biofuel cell consisting of ZnChl-e6 adsorbed on nanocrystalline TiO2 layer coated onto optical transparent conductive glass electrode (OTE) as an anode, platinum-coated OTE as a cathode, and the solution containing sucrose, invertase, glucose dehydrogenase (GDH) and NAD+ as a fuel is studied. The ISC and VOC values of this cell are 9.0 mA cm2 and 415 mV, respectively. The maximum peaks in the photocurrent action spectrum of this cell are observed at 400 and 800 nm. The IPCE values at 400 and 800 nm are estimated to be ca. 17.3% and 10.6%, respectively. As the conversion efficiency of photon-tocurrent and cell performance are still low; however, new type of sucrose–O2 biofuel cell operated with photoenergy is
ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y
developed by using the visible and near IR photosensitization of ZnChl-e6 on nanocrystalline TiO2 film electrode.
Acknowledgements This work was partially supported by a special fund from Venture Business Laboratory of Oita University, JFE 21st Century Foundation, Secom Science and Technology Foundation and TEPCO Research Foundation. R E F E R E N C E S
[1] Wang D, Czernik S, Chornet E. Production of hydrogen from biomass by catalytic steam reforming of fast pyrolysis oils. Energy Fuels 1998;12(1):19–24. [2] Minowa T, Inoue S. Hydrogen production from biomass by catalytic gasification in hot compressed water. Renew Energ 1999;16(1–4):1114–7. [3] Rustamov VR, Abdullayev KM, Aliyev FG, Kerimov VK. Hydrogen formation from biomass using solar energy. Int J Hydrogen Energy 1998;23(8):649–52. [4] Helena LC, Overend RP. Biomass and renewable fuels. Fuel Process Technol 2001;71(1–3):187–95. [5] Barbosa MJ, Rocha JM, Tramper J, Wijffels RH. Acetate as a carbon source for hydrogen production by photosynthetic bacteria. J Biotechnol 2001;85(1):25–33. [6] Garcia L, French R, Czernik S, Chornet E. Catalytic steam reforming of bio-oils for the production of hydrogen: effects of catalyst composition. Appl Catal A Gen 2000;201(2): 225–39. [7] Woodward J, Orr M, Cordray K, Greenbaum E. Enzymatic biohydrogen production. Nature 2000;405:1014–5. [8] Inoue T, Kamachi T, Okura I. Photoinduced hydrogen evolution from glucose using Zn porphyrin as a photosensitizer. Porphyrins 1999;8(1):29–32. [9] Saiki Y, Amao Y. Visible light induced biohydrogen production from sucrose using the photosensitization of Mg chlorophyll-a. Bioconjugate Chem 2002;13(4):898–901. [10] Takeuchi Y, Amao Y. Biohydrogen production from sucrose using the visible light sensitization of artificial Zn chlorophyll-a. Bioconjugate Chem 2003;14(1):268–72. [11] Mano N, Mao F, Heller A. Characteristics of a miniature compartment-less glucose–O2 biofuel cell and its operation in a living plant. J Am Chem Soc 2003;125(21):6588–94.
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[12] Kashiwada A, Nishino N, Wang ZY, Nozawa T, Kobayashi M, Nango M. Molecular assembly of bacteriochlorophyll a and its analogues by synthetic 4a-helix polypeptides. Chem Lett 1999;28(12):1301–2. [13] Kashiwada A, Watanabe H, Tanaka T, Nango M. Molecular assembly of zinc bacteriochlorophyll a by synthetic hydrophobic 1a-helix polypeptides. Chem Lett 2000;29(1):24–5. [14] Iida K, Ohya N, Kashiwada A, Mimuro M, Nango M. Characterization of the light-harvesting polypeptide/bacteriochlorophyll a complex isolated from photosynthetic bacteria by the linear dichroism spectra. Bull Chem Soc Jpn 2000;73(1):221–9. [15] Iida K, Kiriyama H, Fukai A, Konings WN, Nango M. Twodimensional self-organization of the light-harvesting polypeptides/BChl a complex into a thermostable liposomal membrane. Langmuir 2001;17(9):2821–7. [16] Nagata M, Nango M, Kashiwada A, Yamada S, Ito S, Sawa N, et al. Construction of photosynthetic antenna complex using light-harvesting polypeptide-a from photosynthetic bacteria, R. rubrum with zinc substituted bacteriochlorophyll a. Chem Lett 2003;32(3):216–7. [17] Kamat PV, Chauvet JP, Fessenden RW. Photoelectrochemistry in particulate systems. 4. Photosensitization of a titanium dioxide semiconductor with a chlorophyll analog. J Phys Chem 1986;90(7):1389–94. [18] Kay A, Gra¨tzel MJ. Artificial photosynthesis. 1. Photosensitization of titania solar cells with chlorophyll derivatives and related natural porphyrins. J Phys Chem 1993;97(23):6272–7. [19] Kay A, Humphry-Baker R, Gra¨tzel M. Artificial photosynthesis. 2. Investigations on the mechanism of photosensitization of nanocrystalline TiO2 solar cells by chlorophyll derivatives. J Phys Chem 1994;98(3):952–9. [20] Curri ML, Petrella A, Striccoli M, Cozzoli PD, Cosma P, Agostiano A. Photochemical sensitisation process at photosynthetic pigments/Q-sized colloidal semiconductor heterojunctions. Synth Metal 2003;139(3):593–6. [21] Amao Y, Takeuchi Y. Visible light-operated glucose–O2 biofuel cell. Int J Global Energy Issues 2007;28(2–3):295–303. [22] Nakade S, Kambe S, Kitamura T, Wada Y, Yanagida S. Effects of lithium ion density on electron transport in nanoporous TiO2 electrodes. J Phys Chem B 2001;105(38):9150–2. [23] Amao Y, Yamada Y, Aoki K. Preparation and properties of dye-sensitized solar cell using chlorophyll derivative immobilized TiO2 film electrode. J Photochem Photobiol A Chem 2004;164(1–3):47–51. [24] Amao Y, Yamada Y. Near-IR light-sensitized voltaic conversion system using nanocrystalline TiO2 film by Zn chlorophyll derivative aggregate. Langmuir 2005;21(7):3008–12.
33 (2008) 2845 – 2849
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Visible light-operated saccharide–O2 biofuel cell based on the photosensitization of chlorophyll derivative on TiO2 film Yutaka Amao, Yumi Takeuchi Department of Applied Chemistry, Oita University, Dannoharu 700, Oita 870-1192, Japan
art i cle info
ab st rac t
Article history:
The visible light-operated saccharide–O2 biofuel cell consisting of zinc chlorin-e6 (ZnChl-e6)
Received 7 December 2007
adsorbed on nanocrystalline TiO2 layer coated onto optical transparent conductive glass
Received in revised form
electrode (OTE) as an anode, platinum-coated OTE as a cathode, and the fuel solution
19 March 2008
containing sucrose as a saccharide, invertase, glucose dehydrogenase (GDH) and NAD+ is
Accepted 20 March 2008
studied as a new type biofuel cell. The short-circuit photocurrent (ISC) and the open-circuit
Available online 8 May 2008
photovoltage (VOC) of this cell are 9.0 mA cm2 and 415 mV, respectively. The peaks in the
Keywords: Biofuel cell Bioresource Chlorophyll Titanium oxide Photosynthesis
1.
photocurrent action spectrum of this cell are observed at 400 and 800 nm and the incident photon-to-current efficiency (IPCE) values at 400 and 800 nm are estimated to be ca. 17.3% and 10.6%. Thus, a new type of visible light-operated saccharide–O2 biofuel cell with the visible and near IR photosensitization of ZnChl-e6 molecules on nanocrystalline TiO2 film electrode is accomplished. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
Introduction
Energy utilization of the biomass resources is important in the environmental science and the development of energy source research fields [1–3]. Some renewable biomass resources based on the saccharides are starch, cellulose, sucrose, lactose and so on. These oligo- and poly saccharides are hydrolyzed to form monosaccharides such as glucose. The conversion of glucose to hydrogen will be a useful biomass resources utilization. Some studies on the hydrogen production from glucose using the enzymatic pathway have been reported [4–6]. The hydrogen production from glucose with the combination of the glucose dehydrogenase (GDH) and the hydrogenase, which catalyzes the proton reduction, has been reported [7]. Visible light-induced hydrogen production systems from the glucose are also developed with the photosensitizer such as zinc porphyrin and Mg chlorophyll-a in the
reserved.
above-mentioned system [8]. We previously reported the enzymatic photo-induced hydrogen production from sucrose using the photosensitization of Mg or Zn chlorophyll-a as shown in Scheme 1 [9,10]. On the other hand, the biofuel cells using biomass have attracted much attention in recent years [11]. By developing these photo-induced hydrogen production systems in a photoelectrochemical conversion system, a new type of photo-operated saccharide–O2 biofuel cell can be developed. Many studies on the photoelectrochemical conversion system using photosynthesis dyes chlorophyll or its derivative assembled electrode have been reported [12–16]. In the photoelectrochemical conversion system, dye-sensitized nanocrystalline TiO2 cells have attracted much attention as low cost conventional solid state photovoltaic conversion devices for visible light utilization [17–20]. In dye-sensitized nanocrystalline TiO2 cells, TiO2 acts as an electron accepter.
Corresponding author. Fax: +81 97 554 7972.
E-mail address: [email protected] (Y. Amao). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.03.034
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Scheme 1 – Visible light-induced hydrogen production system coupling the sucrose hydrolysis with invertase and GDH and hydrogen production with colloidal platinum using the photosensitization of chlorophyll-a (MChl-a; M ¼ Zn or Mg) in the presence of methylviologen (MV2+). As the dye molecule adsorbed TiO2 shows no photocatalytic activity under visible light region, the chemical components are not decomposed with TiO2. Thus, the dye molecule adsorbed nanocrystalline TiO2 electrode also is useful for the photo-operated saccharide-O2 biofuel cell. We previously reported the photo-operated biofuel cell using chlorophyll derivative adsorbed on nanocrystalline TiO2 layer coated onto optical transparent conductive glass electrode (OTE) as an anode, platinum-coated OTE as a cathode, and the solution containing glucose invertase, (GDH) and NAD+ as a fuel [21]. In the view point of biomass utilization, however, the development of photo-operated biofuel cell using oligo- and polysaccaride is desirable. In this photo-operated biofuel cell, glucose formation is one of the most important processes. As mentioned above, we reported the enzymatic photo-induced hydrogen production with the system combination of sucrose hydrolysis with enzyme invertase and the photoredox reaction of Mg or Zn chlorophyll-a, methylvilogen (MV2+) and platinum colloid as shown in Scheme 1 [9,10]. Thus, it is necessary to combine the glucose formation based on the oligo- and polysaccaride hydrolysis with enzyme in photo-operated biofuel cell. In this paper, we describe the visible light-operated saccharide–O2 biofuel cell consisting of zinc chlorin-e6 (ZnChl–e6) adsorbed on nanocrystalline TiO2 layer coated onto OTE as an anode, platinum-coated OTE as a cathode, and the solution containing sucrose, which is most simple oligo saccharide, invertase, GDH and NAD+ as a fuel as shown in Fig. 1.
2.
Experimental section
2.1.
Materials
Chl-e6, GDH from Bacillus sp. and invertase from Yeast are purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan). NAD+ and NADH are purchased from Oriental Yeast Co. Ltd. Hydrogen hexachloplatinate hexahydrate is obtained from Kanto Chemical Co. Ltd (Tokyo, Japan). Titanium dioxide powder (P25) is purchased from Degussa. OTE (10–15 O/square SnO2: fluorine coated) is obtained from Nihon Sheet Glass Co. Ltd. The other chemicals are analytical grade or the highest grade available. One unit of GDH activity is defined as the amount of enzyme that reduced 1.0 mmol NAD+ to NADH by glucose per min. One unit of invertase activity is defined as the amount of enzyme that produced 1.0 mmol glucose by sucrose per min.
Fig. 1 – Visible light-operated sucrose–O2 biofuel cell based on the combination of NAD+ reduction with enzymatic sucrose hydrolysis photosensitization of ZnChl-e6 on nanocrystalline TiO2 layer-coated OTE (anode) and the electrochemical reduction to the water of oxygen on to the platinum-coated OTE (cathode).
2.2.
Synthesis of ZnChl-e6
ZnChl-e6 is synthesized by refluxing Chl-e6 with about 10 times the molar equivalent of zinc acetate in 100 ml methanol at 60 1C for 2 h. The synthesis of ZnChl-e6 is monitored by UV–vis absorption spectrum using Shimadzu Multispec 1500 spectrophotometer. During the reaction, the characteristic absorption bands of ZnChl-e6 at 418 and 638 nm increase and the absorbance at 400, 514 and 660 nm of Chl-e6 decrease gradually. After the mixture is cooled to room temperature, the solvent is removed under vacuum and then the reaction mixture is washed with water to remove the unreacted zinc acetate dihydrate. Finally, ZnChl-e6 is obtained in water as a precipitation. ZnChl-e6 is collected by filtration and washed with water. The purification is performed by recrystallization from water–methanol (5:1) solution.
2.3.
Preparation of TiO2 film electrode
The nanocrystalline TiO2 film is prepared by a similar procedure to that described in the literature [22]. TiO2 powder is dispersed by grinding water and HNO3 aqueous solution. The viscous suspension is spread onto OTE (1 5 cm2) at room temperature using scotch tape as a spacer. A thin film is obtained by raking off the excess of suspension with a glass rod. After the tape is removed and the plate is dried using hot plate at 80 1C for 30 min, this plate is annealed at 450 1C for
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30 min under ambient condition to form a nanocrystalline TiO2 film electrode. The thickness of the film, determined by using a micron-sensitive calliper, is about 10 mm.
2.4. Preparation of ZnChl-e6 adsorbed nanocrystalline TiO2 electrode ZnChl-e6 adsorbed nanocrystalline TiO2 electrode is prepared as follows. An OTE glass plate with a nanocrystalline TiO2 film is dipped into 0.2 mmol dm3 ZnChl-e6 in methanol solution at room temperature for 6 h. After dipping, the plate is washed with methanol several times and then the plate is dried under vacuum overnight.
2.5.
Preparation of platinum-coated OTE electrode
The platinum-coated OTE electrode is prepared by thermal decomposition of hydrogen hexachloplatinate hexahydrate from 2-propanol solution on an OTE substrate as the following method. An OTE glass plate is dipped into 1.0 mmol dm3 hydrogen hexachloplatinate hexahydrate in 2-propanol solution at room temperature for 30 min. After dipping, the plate is dried at room temperature for 30 min under ambient condition and then is annealed at 380 1C for 30 min under ambient condition. The active area of electrode is 1.0 cm2.
2.6. Characterization of visible-operated sucrose–O2 biofuel cell using ZnChl-e6 adsorbed on nanocrystalline TiO2 film Photocurrent–photovoltage characteristic of the ZnChl-e6 adsorbed on TiO2 electrode is measured with a sandwichtype cell. The working electrode with the ZnChl-e6 adsorbed on TiO2 film is gently squeezed together with a platinumcoated OTE glass electrode (counter electrode) using spring and irradiated from the substrate side of working electrode. The anodic solution consists of 0.1 mol dm3 sucrose, five units invertase, 3.5 mmol dm3 NAD+, five units GDH and 0.1 mol dm3 KCl in 50 mmol dm3 in 50 ml of 10 mmol dm3 potassium phosphate buffer (pH 7.0). The dissolved oxygen in anodic solution is used as cathodic reaction of oxygen to water. A solar simulator (YSS-40, Yamashita Denso) is used as a light source (A.M. 1.5 100 mW cm2) for the photocurrent and photovoltage characteristics with the two digital multimeter with model 2000-J (Keithley) as a current meter and model 34401A (Agilent) as a voltage meter, respectively. To prevent the direct photoexcitation of TiO2, wavelengths of less than 420 nm were blocked with a cut-off filter (SCF-50S-42L Sigma Koki Co., Ltd.). The photocurrent and photovoltage are changed using 500-O variable resistor. The active electrode area is typically 0.25 cm2. The fill factor (FF) is defined by FF ¼ IPhðmaxÞ VPhðmaxÞ =ISC VOC
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defined by the following equation: Z ¼ ISC VOC FF=Pin
(2)
Here Pin is the power of incident white light. A 400 W xenon lamp with a monochromator is used as light source for photocurrent action spectra measurements. The cell is operated in the short-circuit mode. As the ZnChl-e6 has the absorption band in the region between 400 and 900 nm, the incident photon-to-current conversion efficiency (IPCE) values are determined in this region. The IPCE is then calculated according to the following equation: IPCE ¼ 1240iph ðmAÞ=PðmWÞlðnmÞ
(3)
where iph and P are the photocurrent and power of the incident radiation per unit area and l is the wavelength of the monochromatic light. The maximums and the minimum values of light intensity between 400 and 900 nm are 110 and 24 mW cm2, respectively.
3.
Results and discussion
3.1.
Photocurrent–photovoltage characteristics
Fig. 2 shows the photocurrent–photovoltage characteristic of a sandwich cell irradiated with 100 mW cm2. The shortcircuit photocurrent (ISC) is 9.0 mA cm2, and the open-circuit photovoltage (VOC) is 415 mV, respectively. The power output of cell as a function of photovoltage is shown in Fig. 3. The maximum power is estimated to be 0.028 mW cm2. FF and Z values calculated by Eqs. (1) and (2) are 7.4 104 and 0.000028%, respectively. In contrast, ISC and VOC under the dark condition are ca. zero. Thus, this cell is operated with the visible-light photosensitization of nanocrystalline TiO2 films by ZnChl-e6. We already reported the photo-operated glucose–O2 biofuel cell. In the photo-operated glucose–O2 biofuel
(1)
where IPh(max) and VPh(max) are the photocurrent and photovoltage for maximum power output (Pmax) and ISC and VOC are the closed-circuit photocurrent and open-circuit photovoltage. The overall photoenergy conversion efficiency (Z) is
Fig. 2 – Photocurrent–photovoltage characteristic sucrose–O2 biofuel cell under 100 mW cm2 condition.
of
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Fig. 4 – Photocurrent action spectra of sucrose–O2 biofuel cell as a function of wavelength (400–900 nm). Fig. 3 – Power output of sucrose–O2 biofuel cell as a function of photovoltage under light intensity of 100 mW cm2 condition.
cell, FF and Z values are 4.0 103 and 0.0021%, respectively. Thus, the cell performance slightly decreases by combination of sucrose hydrolysis in photo-operated biofuel cell. To investigate the effect of oxygen concentration on the photocurrent–photovoltage characteristics of the cell, the oxygen concentration in the anodic solution is changed. By using the argon-saturated anodic solution, the VOC ( ¼ 420 mV) value is almost the same as that of using the anodic solution including dissolved oxygen. On the other hand, the ISC value is gradually decreased with irradiation time under argonsaturated condition. In contrast, the VOC and ISC values slightly increase using the oxygen-saturated anodic solution. These results show that photo-operated sucrose–O2 biofuel cell based on the visible-light photosensitization of nanocrystalline TiO2 films by ZnChl-e6 is developed. However, the conversion efficiency of photon-to-current is still low.
3.2.
metal-free and ZnChl-e6 adsorbed on a nanocrystalline TiO2 film electrode. The photocurrent action spectrum of cell using metal-free Chl-e6 is similar to that of the UV–vis absorption spectrum in methanol solution and the maximum peak of photocurrent action and UV–vis spectra is 670 nm [23]. However, the photocurrent action spectrum of cell using ZnChl-e6 is shifted to near IR region compared with that of the UV–vis absorption spectrum in methanol solution and the maximum peak of photocurrent action and UV–vis spectra is 780 nm [24]. This result indicates that ZnChl-e6 molecules are aggregated on a nanocrystalline TiO2 film electrode and the absorption band is shifted to near IR region. This result indicates that ZnChl-e6 molecules are aggregated such as bacteriochlorophyll dimer on a nanocrystalline TiO2 film electrode and the absorption band is shifted to red region. Thus, the peak of photocurrent action spectrum of cell based on the visible-light photosensitization of nanocrystalline TiO2 films by ZnChl-e6 is shifted to red region compared with that of UV–vis absorption spectrum in methanol solution.
Photocurrent action spectrum
Fig. 4 shows the photocurrent action spectrum of a sandwich cell, where the IPCE is plotted as a function of wavelength. The maximum peaks in the photocurrent action spectrum of the cell are observed at 400 and 800 nm and the IPCE values at 400 and 800 nm are estimated to be ca. 17.3% and 10.6%. In the photo-operated glucose–O2 biofuel cell, the IPCE values at 400 and 800 nm are estimated to be ca. 7.5% and 5.0%. Thus, the IPCE values at 400 and 800 nm slightly increase by a combination of sucrose hydrolysis in the photo-operated biofuel cell. In contrast, the maximum peaks of UV–vis absorption spectrum in methanol solution are 413 and 638 nm. The peak in the photocurrent action spectrum is shifted to red region compared with that of UV–vis absorption spectrum obtained in a methanol solution. We previously reported the solar cell based on the photosensitization of
4.
Conclusion
In this work, visible light-operated sugar–O2 biofuel cell consisting of ZnChl-e6 adsorbed on nanocrystalline TiO2 layer coated onto optical transparent conductive glass electrode (OTE) as an anode, platinum-coated OTE as a cathode, and the solution containing sucrose, invertase, glucose dehydrogenase (GDH) and NAD+ as a fuel is studied. The ISC and VOC values of this cell are 9.0 mA cm2 and 415 mV, respectively. The maximum peaks in the photocurrent action spectrum of this cell are observed at 400 and 800 nm. The IPCE values at 400 and 800 nm are estimated to be ca. 17.3% and 10.6%, respectively. As the conversion efficiency of photon-tocurrent and cell performance are still low; however, new type of sucrose–O2 biofuel cell operated with photoenergy is
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developed by using the visible and near IR photosensitization of ZnChl-e6 on nanocrystalline TiO2 film electrode.
Acknowledgements This work was partially supported by a special fund from Venture Business Laboratory of Oita University, JFE 21st Century Foundation, Secom Science and Technology Foundation and TEPCO Research Foundation. R E F E R E N C E S
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